2.  What is it ? What is it ?  URI,URN,URL URI,URN,URL  HTTP – methods HTTP – methods  HTTP Request Packets HTTP Request Packets  HTTP Request Headers HTTP Request Headers  Status Header Status Header  HTTP Response Headers HTTP Response Headers  Status Codes Status Codes

3.  Hypertext Transport Protocol  Language of the Web ◦ protocol used for communication between web browsers and web servers  TCP port 80 (443 secure)  RFC 2616 (ver 1.1) BACK

4. URI,URN,URL  Uniform Resource Identifier ◦ Information about a resource  Uniform Resource Name ◦ The name of the resource with in a namespace  Uniform Resource Locator ◦ How to find the resource, a URI that says how to find the resource. BACK

5.  URL ◦ Uniform Resource Locator  protocol (http, ftp, news)  host name (name. domain name)  port (usually 80 but many on 8080)  directory path to the resource  resource name ◦ http://xxx.myplace.com/www/index.html ◦ http://xxx.myplace.com:80/cgi-bin/t.exe BACK

6.  Methods ◦ GET  retrieve a URL from the server  simple page request  run a CGI program  run a CGI with arguments attached to the URL ◦ POST  preferred method for forms processing  run a CGI program  more secure and private …

7.  Methods (cont.) ◦ PUT  Used to transfer a file from the client to the server ◦ HEAD  requests URLs status header only  used for conditional URL handling for performance enhancement schemes  retrieve URL only if not in local cache or date is more recent than cached copy BACK

8.  Sent from client to server  Consists of HTTP header ◦ header is hidden in browser environment ◦ contains:  content type / mime type  content length  user agent - browser issuing request  content types user agent can handle  and a URL Back BACK

9.  Precede HTTP Method requests  headers are terminated by a blank line  Header Fields: ◦ From ◦ Accept ◦ Accept-Encoding ◦ Accept Language Back

10.  “HTTP/1.0 sp code”  Codes: ◦ 1xx - reserved for future use ◦ 2xx - successful, understood and accepted ◦ 3xx - further action needed to complete ◦ 4xx - bad syntax in client request ◦ 5xx - server can’t fulfill good request BACK

11.  Sent by server to client browser  Status Header ◦ Entities  Content-Encoding:  Content-Length:  Content-Type:  Expires:  Last-Modified:  extension-header  Body – content (usually html) Back

12.  200 OK  201 created  202 accepted  204 no content  301 moved perm.  302 moved temp  304 not modified  400 bad request  401 unauthorized  403 forbidden  404 not found  500 int. server error  501 not impl.  502 bad gateway  503 svc not avail Back